14 14-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 7e...

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14-1© 2003 Thomson Learning, Inc.All rights reserved

General, Organic, and General, Organic, and Biochemistry, 7eBiochemistry, 7e

Bettelheim,Bettelheim,

Brown, and MarchBrown, and March

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Chapter 14Chapter 14

Alcohols, Ethers, Alcohols, Ethers, and Thiolsand Thiols

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AlcoholsAlcohols• Alcohol:Alcohol: a compound that contains an -OHOH

(hydroxyl) group(hydroxyl) group bonded to a tetrahedral carbon• methanol, CH3OH, is the simplest alcohol

• Nomenclature1.select the longest carbon chain that contains the -OH

group as the parent alkane and number it from the end that gives the -OH the lower number

2.change the ending of the parent alkane from -ee to -olol and use a number to show the location of the -OH group; for cyclic alcohols, the carbon bearing the -OH group is carbon-1

3.name and number substituents and list them in alphabetical order

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NomenclatureNomenclature

Ethanol(Ethyl alcohol)

1-Propanol(Propyl alcohol)

2-Propanol(Isopropyl alcohol)

1-Butanol(Butyl alcohol)

OH

OH

OHOH

2-Butanol(sec-Butyl alcohol)

2-Methyl-1-propanol(Isobutyl alcohol)

2-Methyl-2-propanol(tert-Butyl alcohol)

OH

Cyclohexanol(Cyclohexyl alcohol)

OHOH

OH

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NomenclatureNomenclature• Problem:Problem: write the IUPAC name for each alcohol

(a) (b)OH OH

(c) (d) OHOH

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NomenclatureNomenclature• Solution:Solution:

(a) (b)OH OH

(c) (d) OHOH

1

22

2 2

4

trans-2-Methylcyclohexanol4-Methyl-2-pentanol

2-Heptanol

7

5

3

2,2-Dimethyl-1-propanol

1

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NomenclatureNomenclature• in the IUPAC system, a compound containing two

hydroxyl groups is named as a dioldiol, one containing three hydroxyl groups as a trioltriol, and so on

• IUPAC names for diols, triols, and so on retain the final "-e" in the name of the parent alkane

• we commonly refer to compounds containing two hydroxyl groups on adjacent carbons as glycolsglycols

CH3CHCH2HO OH

CH2CH2OH OH

CH2CHCH2

OH OHOH1,2-Ethanediol

(Ethylene glycol)1,2-Propanediol

(Propylene glycol)1,2,3-Propanetriol

(Glycerol, Glycerin)

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Physical PropertiesPhysical Properties• Alcohols are polar molecules

• the C-O and O-H bonds are both polar covalent

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Physical PropertiesPhysical Properties• in the liquid state, alcohols associate by hydrogen

bonding

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Physical PropertiesPhysical Properties• bp increases as MW increases• solubility in water decreases as MW increases

CH3CH2CH2OH

CH3CH2CH2CH3

CH3OHCH3CH3

CH3CH2OH

CH3CH2CH3

CH3CH2CH2CH2OH

CH3CH2CH2CH2CH3

Structural Formula NameMolecularWeight

bp(°C)

Solubilityin Water

methanol 32 65 infiniteethane 30 -89 insoluble

ethanol 46 78 infinite

propane 44 -42 insoluble

1-propanol 60 97 infinite

butane 58 0 insoluble

8 g/100 g117741-butanol

pentane 72 36 insoluble

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Acidity of AlcoholsAcidity of Alcohols• Alcohols have about the same pKa values as

water; aqueous solutions of alcohols have the same pH as that of pure water

• alcohols and phenols both contain an OH group• phenols are weak acids and react with NaOH and other

strong bases to form water-soluble salts

• alcohols are weaker acids and do not react in this manner

OH NaOHH2O

O-Na+ H2O+

Phenol Sodium phenoxide(a water-soluble salt)

+

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DehydrationDehydration• Dehydration:Dehydration: elimination of a molecule of water

from adjacent carbon atoms gives an alkene• dehydration is most often brought about by heating an

alcohol with either 85% H3PO4 or concentrated H2SO4

• 1° alcohols are the most difficult to dehydrate and require temperatures as high as 180°C

• 2° alcohols undergo acid-catalyzed dehydration at somewhat lower temperatures

• 3° alcohols generally undergo acid-catalyzed dehydration at temperatures only slightly above room temperature

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DehydrationDehydrationCH3CH2OH

H2SO4 CH2=CH2 H2O+Ethanol Ethylene

180°C

OHH2SO4 H2O140°C

Cyclohexanol Cyclohexene

+

CH3CCH3

OH

CH3H2SO4 CH3C=CH2

CH3

H2O50°C

2-Methyl-2-propanol (tert -Butyl alcohol)

2-Methylpropene (Isobutylene)

+

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DehydrationDehydration• when isomeric alkenes are obtained, the alkene having

the greater number of alkyl groups on the double bond generally predominates

• examples:

CH3CH2CHCH3

OH H3PO4 CH3CH=CHCH3 CH3CH2CH=CH2

1-Butene (20%)

2-Butene (80%)

2-Butanol

+-H2O

CH3CHCHCH3OH

CH3 H2SO4CH3C=CHCH3

CH3

CH3CHCH=CH2

CH3

3-Methyl-1-butene2-Methyl-2-butene (major product)

3-Methyl-2-butanol

+-H2O

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Dehydration-HydrationDehydration-Hydration• Acid-catalyzed hydration of alkenes to give

alcohols and acid-catalyzed dehydration of alcohols to give alkenes are competing reactions• the following equilibrium exists

• in accordance with Le Chatelier's principle, large amounts of water favor alcohol formation, whereas removal of water from the equilibrium mixture favors alkene formation

An alkene An alcohol

C CH OH

+ H2Odehydration

hydrationC C

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OxidationOxidation• Oxidation of a 1° alcohol gives an aldehyde or a

carboxylic acid, depending on the experimental conditions• oxidation of a 1° alcohol to a carboxylic acid is carried

out using potassium dichromate, K2Cr2O7, in aqueous sulfuric acid

• it is often possible to stop the oxidation at the aldehyde stage by distilling the mixture; the aldehyde usually has a lower boiling point than either the 1° alcohol or the carboxylic acid

CH3(CH2)6CH2OHK2Cr2O7

CH3(CH2)6CHO K2Cr2O7

H2SO4 H2SO4CH3(CH2)6COH

O

Octanal Octanoic acid1-Octanol

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OxidationOxidation• oxidation of a 2° alcohol gives a ketone

• tertiary alcohols are resistant to oxidationCH3

OH

K2Cr2O7

H2SO4

1-Methyl-cyclopentanol

(no oxidation)

OH

K2Cr2O7

H2SO4 O

2-Isopropyl-5-methyl-cyclohexanone(Menthone)

2-Isopropyl-5-methyl-cyclohexanol(Menthol)

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EthersEthers• The functional group of an etherether is an oxygen

atom bonded to two carbon atoms • the simplest ether is dimethyl ether• the most common ether is diethyl ether

CH3-O-CH3 CH3CH2-O-CH2CH3

Dimethyl ether Diethyl ether

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NomenclatureNomenclature• Although ethers can be named according to the

IUPAC system, chemists almost invariably use common names for low-molecular-weight ethers• common names are derived by listing the alkyl groups

bonded to oxygen in alphabetical order and adding the word "ether”

• alternatively, name one of the groups on oxygen as an alkoxy group

OCH3CH3CH2OCH2CH3

Cyclohexyl methyl ether(Methoxycyclohexane)

Diethyl ether

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NomenclatureNomenclature• Cyclic ether:Cyclic ether: an ether in which one of the atoms

in a ring is oxygen• cyclic ethers are also known by their common names• ethylene oxide is an important building block for the

organic chemical industry; it is also used as a fumigant in foodstuffs and textiles, and in hospitals to sterilize surgical instruments

• tetrahydrofuran is a useful laboratory and industrial solvent

Ethylene oxide Tetrahydrofuran (THF)

OO

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Physical PropertiesPhysical Properties• Ethers are polar compounds in which oxygen

bears a partial negative charge and each carbon bonded to it bears a partial positive charge• however, only weak forces of attraction exist between

ether molecules in the pure liquid • consequently, boiling points of ethers are close to

those of hydrocarbons of similar molecular weight• ethers have lower boiling points than alcohols of the

same molecular formula

CH3CH2OH CH3OCH3

bp -24°C

Ethanol

bp 78°C

Dimethyl ether

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Reactions of EthersReactions of Ethers• Ethers resemble hydrocarbons in their resistance

to chemical reaction• they do not react with oxidizing agents such as

potassium dichromate

• they do not react with reducing agents such as H2 in the presence of a transition metal catalyst

• they are not affected by most acids or bases at moderate temperatures

• Because of their general inertness and good solvent properties, ethers such as diethyl ether and THF are excellent solvents in which to carry out organic reactions

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ThiolsThiols• Thiol:Thiol: a compound containing an -SHSH (sulfhydryl)(sulfhydryl)

groupgroup• the most outstanding property of low-molecular-weight

thiols is their stench• they are responsible for smells such as those from

rotten eggs and sewage• the scent of skunks is due primarily to these two thiols

CH3CH=CHCH2SH

CH3

CH3CHCH2CH2SH3-Methyl-1-butanethiol2-Butene-1-thiol

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Thiols - NomenclatureThiols - Nomenclature• IUPAC names are derived in the same manner as

are the names of alcohols• to show that the compound is a thiol, the final -e-e of the

parent alkane is retained and the suffix -thiol-thiol added

• Common names for simple thiols are derived by naming the alkyl group bonded to -SH and adding the word "mercaptanmercaptan"

CH3CH2SH

CH3

CH3CHCH2SHEthanethiol

(Ethyl mercaptan)2-Methyl-1-propanethiol

(Isobutyl mercaptan)

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Physical PropertiesPhysical Properties• Because of the small difference in

electronegativity between sulfur and hydrogen (2.5 - 2.1 = 0.4), an S-H bond is nonpolar covalent • thiols show little association by hydrogen bonding• thiols have lower boiling points and are less soluble in

water and other polar solvents than alcohols of similar molecular weight

11778

65

1-butanolethanol

methanol

9835

6

1-butanethiolethanethiol

methanethiol

bp (°C)Alcoholbp (°C)Thiol

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Reactions of ThiolsReactions of Thiols• Thiols are weak acids (pKa 10), and are

comparable in strength to phenols• thiols react with strong bases such as NaOH to form

water-soluble thiolate salts

CH3CH2SH NaOHH2O

CH3CH2S-Na+ H2O+Ethanethiol

(pKa 10)

+Sodium

ethanethiolate

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Reactions of ThiolsReactions of Thiols• The most common reaction of thiols in biological

systems is their oxidation to disulfides, the functional group of which is a disulfidedisulfide (-S-S-)(-S-S-) bondbond • thiols are readily oxidized to disulfides by O2

• they are so susceptible to oxidation that they must be protected from contact with air during storage

• disulfides, in turn, are easily reduced to thiols by several reducing agents.

2HOCH2CH2SH HOCH2CH2S-SCH2CH2OHA disulfide

oxidationreductionA thiol

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Important AlcoholsImportant AlcoholsCO

CO2H2 CH3OH

O2 CH2O

CH3COOHCoalor

methane Carbonmonoxide

Methanol

Acetic acid

Formaldehydeoxidation

catalyst

CH2=CH2

O2

H2O, H2SO4

OH2C CH2

CH3CH2OH

H2O, H2SO4

H2SO4

HOCH2CH2OH

CH3CH2OCH2CH3

Ethylene

Ethanol

Ethylene glycol

180°C Diethyl ether

catalystEthylene

oxide

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Important AlcoholsImportant AlcoholsH2O, H2SO4

CH3CH=CH2

CH3CHCH3

OH

HOCH2CHCH2OHOHPropene

2-Propanol

Glycerin, glycerol

several steps

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End End Chapter 14Chapter 14

Alcohols, Ethers, and ThiolsAlcohols, Ethers, and Thiols

14 14-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 7e...

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